X-ray generating assembly and system

ABSTRACT

An X-ray generating tube of a type employing a target of material disposed to be struck by high energy electrons to develop X-rays employs a hollow beam of electrons impinging upon the target. An oxide type, highly emissive cathode assembly provides much higher emission at lower temperatures and with prolonged life against the normally deleterious effects of ion bombardment using an ion collector surface within the hollow beam to intercept ions attracted into the beam in favor of the actively emitting portion of the cathode. A control grid serves to form the hollow beam, provide the shield, and when connected to a pulse generator, serves to modulate or pulse the beam.

United States Patent Gralenski 1 Sept. 12, 1972 [54] X-RAY GENERATING ASSEMBLY AND [21] Appl. No.: 854,393

[52] US. Cl. ..313/57, 313/59, 313/240, 313/293, 313/333 [51] Int. Cl .1101] 35/06, HOlj 35/14, H01j35/16 [58] Field of Search.....3l3/55, 57, 58, 59, 56, 107.5, 313/333, 106, 82, 240, 293

[56] References Cited UNITED STATES PATENTS 1,684,263 9/1928 Coolidge ..313/57 X 2,046,808 7/1936 Bouwers et a1. ..3l3/58 X 2,864,965 12/1958 Wang ..313/82 0 RETURN FIN BIAS ADJUST Feinstein ..3l3/82 X Koziak ..3l3/82 Primary Examiner-Robert Segal Assistant Examiner-Darwin R. Hostetter Attorney-Flehr, Hohbach, Test, Albritton & Herbert [S 7] ABSTRACT An X-ray generating tube of a type employing a target of material disposed to be struck by high energy electrons to develop X-rays employs a hollow beam of electrons impinging upon the target. An oxide type, highly emissive cathode assembly provides much higher emission at lower temperatures and with prolonged life against the normally deleterious effects of ion bombardment using an ion collector surface within the hollow beam to intercept ions attracted into the beam in favor of the actively emitting portion of the cathode. A control grid serves to form the hollow beam, provide the shield, and when connected to a pulse generator, serves to modulate or pulse the beam.

1 Claim, 5 Drawing Figures POWER SUPPLY PATENTED SEP 12 :972

SHEEI 2 0f 2 INVENTOR.

NlCHOLAS M. GRALENSKI 741a,, M41204, m, awn) Mu ATTORNEYS X-RAY GENERATING ASSEMBLY AND SYSTEM BACKGROUND OF THE INVENTION This invention pertains to an X-ray generating device such as a vacuum tube, and more particularly to an X- ray tube suitable for use in applications requiring extraordinarily high beam current, for example, on the order of 200 milliamps.

In an X-ray tube electrons are accelerated by a high voltage applied between a cathode and an anode whereby the electrons move rapidly into the anode. When the electrons collide with the anode, X-rays are produced. Typically, this bombardment is conducted in a relatively high vacuum environment such as produced within conventional vacuum tube envelopes. However, within the envelope, limited amounts of gas exist whereby during operation of the tube positively charged ions are created. These ions are, of course, immediately attracted to the negatively charged cathode and serve to bombard the cathode causing erosion and contamination of a type which serves to limit the life of cathodes employed in X-ray tubes. This erosion and contamination increase rapidly with an increase in beam current defined between the cathode and anode.

Accordingly, while it is known that certain oxide coated cathodes provide a highly emissive electron source, they are quite sensitive and deteriorate rapidly under ion bombardment of a type as would be experienced in the above environment.

Conventional cathode materials having greater resistance to contamination and deterioration have been used, such as tungsten, but are typically required to be operated with considerable power. Beam currents on the order of 100 to 400 milliamperes, while desirable, therefore, have been difficult, if not impossible, to obtain.

I SUMMARY OF THE INVENTION AND OBJECTS In general, an X-ray generating device has been provided of a type employing a target of material disposed to be struck by high energy electrons so as to develop X-rays. A cathode formed with an electron emitting portion adapted to be charged negatively with respect to the target so as to create a flow of electrons from that emitting portion serves to form a flow of electrons in the shape of a hollow elongated beam impinging upon the target. Means for focusing the annular elongated flow of electrons enhances the control of the beam to ensure that the beam impinges upon the target. Means forming an ion impact zone within the beam in the region of the cathode portion serves to absorb the impact of ions entering the beam in favor of their impinging upon the electron emitting portion of the cathode.

In addition to the above, means are provided for pulsing the electron beam under control of an annular grid element encircling the electron emitting surface of the cathode.

In general, it is an object of the present invention to provide an improved X-ray tube and system for modulating the operation thereof.

It is another object of the invention to provide an X- ray tube having an enhanced beam current characteristic increased by several orders of magnitude.

It is yet another object of the invention to provide an X-ray generating device of a type characterized by an oxide-coated cathode arranged to provide satisfactory functioning over a relatively long life.

The foregoing and other objects will become more readily apparent from the detailed description of a preferred embodiment according to the invention when considered in conjunction with the following drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a side elevation view of an X-ray tube and system according to the invention.

FIG. 2 is a transverse section view taken along the line 22 of FIG. 1.

FIGS. 3 and 4 are respectively diagrammatic side elevation and end views showing the relationship between cathode and grid.

FIG. 5 is a further enlarged detailed view along the line 5-5 of FIG. 3 diagrammatically showing the relationship between the cathode and grid.

DESCRIPTION OF A PREFERRED EMBODIMENT An X-ray generating tube assembly 11 comprises an elongated evacuated envelope 12 of glass or ceramic material. At one end of envelope 12 a cylindrically shaped metallic insert 13 of a type such as Kovar suitable for forming a seal with glass or ceramic as used in envelope 12, forms a reentrant opening 14 into which a target terminal assembly 16 can be mounted and sealed.

Assembly 16 comprises a cylindrical button 17 or section of suitable material such as tungsten to be bombarded by electrons to form X-rays. Button 17 is formed with a planar face 18 disposed transversely of the axis of button 17 and oriented at an angle on the order of 76 thereto.

Button 17 is brazed inside of the metallic insert 13 and a suitably sized anode terminal rod 19 is brazed to the back of button 17. Rod 19 is preferably of a highly heat-conductive material so as to quickly dissipate heat from button 17 and form a heat sink therefor.

During the glassing operation, where envelope 12 is formed of glass, the thinness and poor thermal conductivity of the Kovar tubing 13 prevents heat from being excessively lost to other portions of assembly 16. At the same time, rod 19 can be highly heat conductive to allow a rapid heat drain from button 17 during tube operation. In addition, cooling fins or wires can be attached to rod 19 by suitable means such as soldering after the assembly 1 l is completely vacuum sealed.

Terminal rod 19 is adapted to be coupled to the positive terminal of a relatively high voltage source on the order, for example, of 40,000 volts so as to form target assembly 16 as the anode of tube assembly 11. The other side of power supply 21 is directly coupled via line 22 to a cathode terminal 23 carried in the end of envelope 12. Cathode terminal 23 is electrically cou' pled to cathode assembly 24, now to be described.

A generally cylindrical metallic body 26 encloses a ceramic insulator and support plug 27. Plug 27 is held in place within body 26 by means of a retaining lip 28 bent around the rear edge of plug 27 on one side and by means of the generally conically shaped hollow annular enclosure 29 on the other.

Coaxially of plug 27 a relatively thin, cylindrical metal thermally-insulating sleeve 31 is spot welded at its rear end to a projecting collar portion 29a of enclosure 29 and serves, at its other end, to support a heater housing 32 encircling the coils of a heater element 33. Heater 33 is connected by leads 34, 36, passing through plug 27 and subsequently by attachment to respective ones of the pins 37, 38. Accordingly, a rivet 39 anchors the inner end of an L-shaped support leg 41 while a connecting strap 42 extends between leg 41 and pin 37. A 6 volt power supply 43 or other suitable low voltage source can be used to energize the heater element 33.

The inner end of cathode assembly 24 carries an electron emitting portion adapted to be charged negatively relative to the target assembly 16 to form a flow of electrons from the cathode emitting portion to the target. The electron emitting portion of cathode assembly 24 comprises an annular oxide-coated surface portion in the form of the edge of lip 44 of cup 46. Cup 46 consists of a suitable metallic material upon which there is deposited an oxide coating of a highly electron emitting material such as strontium, calcium, or barium, or a mixture thereof, deposited around the edge of lip 44. The remainder of cup 46 can be made of conventional cathode nickel.

Cup 46 seats snugly within the cylindrical end of housing 32 directly adjacent heater element 33. A perforated triangular plate 47 is formed with a hole or circular opening encircling, in closely spaced relation, the end margin of housing 32. The outer three corners of plate 47 are secured, by suitable means such as spot welding, to the tabs 48 formed on the end of body 26.

As thus arranged, the triangular plate 47 provides rigidity to the distal end of housing 26 and further serves to align and retain within relatively close tolerances the position of cup 46 with its electron emitting annular surface portions 44.

The edge of lip 44 forms a flow of electrons from cathode assembly 24 to target assembly 16. Thus, the electron emitting oxide-coated surface portion of lip 44 of the cathode is directed toward the target in a manner to form a hollow annular electron beam.

For additional focusing of the beam onto target assembly 16, a grid element 49 is carried by insulators 51, attached to plate 47 and grid 49, in coaxial relation to the oxide-coated electron emitting surface of lip 44.

Grid element 49 includes relatively broad surface portions disposed transversely to the axis of the electron beam 50 and includes tapered surface portions serving to direct the emitted electrons as a hollow beam of electrons toward the target.

Thus, grid element 49 includes an annular opening comprised of three arcuate sections 52 of a circle disposed coaxially of the center of grid element 49. Between each adjacent pair of arcuate circle sections 52 there remains a radial support 53 serving to hold the central ion shield 54 coaxially of grid 49.

Grid 49 is preferably made of refractory material whereby it will stand up under continued bombardment by ions generated within envelope 12 during operation.

As thus arranged, ions, attracted into the central portion of the electron beam 50, tend to impinge upon the shield 54 in favor of their impinging upon the electron emitting edge of lip 44. In this way, the life of the oxidecoated emitting edge is substantially extended.

Grid 49 influences the electron emission of beam 50 and serves to direct it as an annular hollow beam by means of the focusing action of the surfaces of grid 49 remote from cup 46. An outer annular surface 56 slopes inwardly toward the center of grid 49 at a Pierce angle, for example, on the order of 225. A second annular surface 57 tapers rearwardly and radially outwardly of the center of grid 49 at a similar Pierce angle. The two surfaces 56, 57 are spaced slightly to define the circular opening centrally of grid 49 through which electrons will be emitted from the cathode assembly 24.

Means are provided for biasing grid 49 to cathode potential or slightly above, as desired, and also for pulsing the electron beam 50.

Accordingly, a variable resistance 58 taps into line 22 to provide a variable voltage drop between the potential on line 22 (and hence cathode 24) and the potential of grid 49. The wiper 58a is directly connected via a diode 59, poled in a direction to pass current to grid 49, via line 61 (entering through one of the pins disposed through the end of envelope 12). Line 61 is directly attached to grid 49 as shown.

Accordingly, it is readily evident that grid 49 can be adjusted to the potential of cathode assembly 24 or arranged at a variable bias condition by the bias adjusting resistor 58.

Means are provided for pulsing or modulating the operation of electron beam 50 by means of the pulse generator 62 connected directly into line 61 whereby upon application of a pulse to line 61, the electron beam 50 is gated to pass through grid 49 onwardly to target assembly 16.

In operation, assuming that heater element 33 is coupled to power supply 43 and cathode assembly 24 connected to power supply 21 while the other side of power supply 21 is coupled to the target assembly 16, an annular, hollow electron beam will be directed from the oxide-coated portions of cup 46 to impinge upon the tungsten surface of button 17. The angle of the face of button 17 relative to the axis of beam 50 serves to minimize absorption of X-rays by the anode in the region laterally to the side of the axis of beam 50. Ions created within the gas remaining in envelope l2 become positively charged and, by the attraction of beam 50, are accelerated into the interior of beam 50 whereby they seek the most negative portion of cathode assembly 24. These ions then move to bombard the ion impingement shield formed by surface 57 within beam 50 in the region of the emitting portion of cathode assembly 24. Thus, surface 57 serves to intercept ions attracted into the beam 50 in favor of their striking the cathode emitting lip 44 of cup 46.

The potential on grid 49 relative to the potential of the electron emitting portion 44 of cathode 24 and also the Pierce angle efi'ect provided by the sloping surfaces 56, 57 serves to form a hollow beam 50 which can be variously adjusted so as to direct the beam 50 onto the angled surface of button 17.

The foregoing assembly has been observed to obtain a high beam current several orders of magnitude higher than those previously achieved heretofore with conventional apparatus; a long life whereby the oxide-coated cathode is not consumed or contaminated prematurely; and the beam current can be modulated relatively easily with a relatively low voltage on the order, for example, of volts as distinguished from attempting to control the pulsing of the beam by controlling the high cathode portion substantially in the plane of the shielding means and forming an annular opening therethrough in conjunction with said shielding means, the edge of said cup-shaped member being disposed in protective relation from ion impingement at said annular opening defined between said grid and said shielding means, and means for applying a charge to said shielding means and said grid element which is less than the charge on the anode for focusing said beam upon said target. 

1. In an X-ray generating device, a target of material disposed to be struck by electrons to generate X-rays, a cathode comprising an annular cup-shaped member adapted to be charged negatively with respect to said target to emit electrons thereto as an annular elongated beam, shielding means disposed within said beam between said target and said cathode to inhibit ions within said beam from impinging upon said cathode, a grid element disposed in the region of said annular cathode portion substantially in the plane of the shielding means and forming an annular opening therethrough in conjunction with said shielding means, the edge of said cup-shaped member being disposed in protective relation from ion impingement at said annular opening defined between said grid and said shielding means, and means for applying a charge to said shielding means and said grid element which is less than the charge on the anode for focusing said beam upon said target. 